Serpentine traveling wave tube



2581!),851" I v SERPENTINE TRAVELING WAVE TUBE Cassius C. Cutler, Gillette, IN; J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 7 invention relates to microwave devices andmore particularlyto devices which utilize theintr'action between an electronstream anda' tr'aveling electromagnetic wave to secure gain toithe travelingwave.

In such devices; now generallvknownastraveling wave tubes, awave circuitaplurality" of" operating wave lengths long propagates radio frequency electromagnetic waves there'throu'gh at velocities slower than thevelocity of light and an electron stream is projected in the direction of Wave'propagation'throughthe electric field setup by the electric circuit. Byproper adjustment of the velocities of the'electron stream and the propagated wave, nae-mama be made to interact whereby the wave is amplified and the stream is density? andj velocity modulated. In operation; the radio frequency field-ofthe wave circuit acc'elerateselectrons in the stream; giving rise there-' in to an A-C. velocity component which sets up an A.-C convection current component. This in turn set's up a radio frequency field ofits'owrrwhichcombines with the radio frequency field ofthewave circuit. When the-radio frequency wave and electron stream are properly --syn-' chronized, thecumulative actionand reacti'odbetween' the radio frequency-field of' the circuit and the A.-C. current'component in the stream results in a'wavewhich grows in magnitude asit travels along the circuit. 7

Various wave circuits have been proposed hitherto for providing wave propagation -sufliciently slow for interac-, tion with conveniently realized' electronstreams; Practical considerations usuallyfavor an'eleetfon' velocity no faster than one-tenththevelocity*of -li'ght so that the velocity of the traveling wave iii the-direction of the electron stream rnust he' adjustedproportionately. Some of these proposed circuits, such asthe helix; although desira ble' from the standpoint'of'wide band andhigh .gain' characteristics, havepoor dissipative properties which restricttheir power handling capacity, are diflicult of. con struction and assembly, andfalsopose formidable'problerns of impedance matching at 'input and output terminals. Others, such -as"those -of the filter type; are' disadvantageous from the standpoint of limited'bandfwidth and low gain. g p I r V 7 Accordingly; the primafyob ject of the present invention is-to'provide a'novel traveling wavetubeof improveddesign particularly suitable for high power operation over a wide frequency bandwidth one feature of the p're'sentinverition is'a wave circuit comprising a'-waveguide* foldedfbaelt'and forth on itself a plurality of "times forming a serpentine-like" structure. This circuit has simplicity of structure; good dissipative properties, and wide band characteristics. Additionally, this structure facilitates the introduction and derivation ofthewave signals, beingparticularlyadaptable for the convenient admission of a pliir'alitfof s'ignalsfroinseparatesources'for mixin'g' ormo'dulating H Another feature is" a." plurality of distinct electron stre'a'n'i's, each projected along a' separate electron path nited States Pate-ire,

maybe operated either at the same velocity; thereby-serv mg primarily to increase the size of the effective interacting electron stream 'aiid hencethe gain, or at different velocities, staggered to provide a-desired-shapingeither of the frequency-gain orthe gaindevelcharacteristics of the I ttibeb Another feature a is the positioning ofthe regionsof beam traversal of the folded"w-ave=-guide approximatelyan oddnumber of 'quarter wavelengths of the operatingfrequency from= the axisof the serpentine wave guide structure to secure optimum phasing for' the'intera'ctiona The-term axis of the serpentin'e wavefguidestructure is used herein to denote 'aline transverse to the-successive wave guidefol'ds ofthe serpentine structure-andequidistant from' the two ends-ofeaeh individual fold.

in an exemplary embodiment which illustrates features of the invention, the wavecircuit comprises a-serpenti'ne' waveguide foldedback and forth on itself a pluralityof' times, and the successive folds are' tr-aversed; by a'plurality c t-"electron streams each h'aving' a di s tinct'electr'onpath displaced an odd num'oer of 'quarter' wavelengths "of the operating' frequeucy from tiie'axis of the serpentine wave" guide.

Various other features-and secondary objects will be evident from the following more detailed description taken in 'connection'with the drawings "in which:

Fig. lshows schematically an embodimentof the-in ventionw'hich utilizes 'a singleelectron stream;

[shows schematically another embodiment which utilizesthree distinct electron streams; and

Fig; 3 shows schematically" an embodiment adapted for mixing two input signals.

With'referencenoato thedrawings, in' Fig, ljthere is.

shown a traveling wave tube" 10 incorporating several features of the present invention. As the wave circuit; there is provided a wave guide 11 "a plurality of operating wavelengths long, which, for'example, can'be rectangular in cross section, which 'is folded back and forth'on' itself in -a serpentine fashion so that an'electron stream maybe Wave energy is applied'at the input end 15'which, for example; may besimply a-co'ntinuation' byway 'of a pressure-tight :indow' of a wave guide which isp'art ofa'transmission' system: The amplified wave is derived atthe output'end 16 which siinilarlymay' be made integral byway' of a pressure-tight window, with a wave guide of a transmis sion -system'. 'Thisarrangement makes possible a continuous path'of travelforthe' wave energy and" facilitates the introduction and derivation ofwave energy to the tube, obviating the need "for complex" couplingi'elements. An electron gun'll2 isprovided beyo'n'd' the first'fold'of the wave circuit for projecting-an electron stream transverse to the folds of the waveguide: Althouglfan electron gun customarily includes a heater-element, an=electron emis sive cathode, and vari'ous' focusing and acceI'erating'eIec trodes; for purposes of simplicitythesehave not been shown'in' detail here. Bey'ondthe last'fold of the wave circuit and in target relationship" with the" electron gun; there is positioned the collector electrode 13 for collecting the spent electronsafter their traversal of the wave circuit. In accordance with one feature of the invention, the electron path 17 definedby the electron gun and the collector electrode extends parallel'to but displaced transversely approximatelyone-quarter the wavelength of the operating frequency from the axis 18of the. serpentine for traversing the serpentinewave circuitand interactingwith the electric field thereinf These various streams wave guide. Accurate alignmentof the electron flow with the electron path is .e'nhancedby providing an axial magnetic field by suitable. magnetic flux protlur's'ingv means external to thetube' ("not shown here). 'To' minimize disturbance of this axial magnetic field, the main tube con- Patented Oct. 22, 1957 V struction should preferably be of a suitable non-magnetic conductor, for example, copper.

The wave to the input 15 is applied to have a transverse electric field in which the electric lines of force have components transverse to the axis of the guide and in a direction to be parallel to the direction of electron flow. This same type of field is produced in the wave guide by the electron flow transverse through the guide. If the time required for the electrons to pass from the first to the last guide section is substantially the same as that required for the electromagnetic wave, and if successive wave guide traversals are properly phased, the two fields will interact in a manner to produce amplification of the wave in accordance with conventional traveling wave type interaction. The transit time of the electron stream is determined by the average electron velocity which is related to the accelerating beam voltage provided by the voltage source 20. The transit time of the wave depends on the length of the serpentine path. By suitably relating the length and spacing of the folds and the accelerating beam voltage, the desired conditions for transit time can be satisfied.

For cumulative interaction it is of course necessary that each electron encounters substantially the same field phase condition in each successive region of wave guide traversal. This requirement is not automatically met both because the folding of the wave guide reverses the sense of the electric field of the wave guide with respect to the electron stream and because of the electrically long wave path between successive regions of beam interception. A solution to this difiiculty is bad by causing the wave transit time between successive regions of beam traversal to differ by one-half period from the transit time required by the electron stream. In accordance with one feature of the present invention, this is achieved by positioning the electron path substantially one-quarter of a wavelength of the midband operating frequency from the axis or center of the serpentine wave guide. As a result the time between successive traversals differs alternately by plus and minus one-half periods, so that there is no cumulative phase difference in traversing many sections and therefore amplification is realized over a wide band. It should be evident that substantially the same effect can be realized by making the time between successive traversals differ alternately by any odd multiple of a half period, in which case the electron path is positioned a corresponding odd number of quarter wavelengths away from the axis of the serpentive wave guide to secure the desired phasing.

As a condition for good broad-band amplification, it is important that the various wave guide bends do not introduce mismatch effects over the operating range. It can be shown that there is a particular choice of length a at the bends with square corners which permits a wide band match. In particular instances, it may be preferable in order to minmize mismatch elfects of the bends to utilize other types of bends, such as curved bends, consistent with the principles set forth herein.

In order to obtain approximate synchronisrn between the wave and stream velocities certain circuit proportions must be satisfied. In particular, it is important that:

where L and L2 represent the lengths of two successive wave paths between successive traversals, :1 represents the center-to-center separation of successive traversals along the electron path, 11 and v are the wave and electron velocities, respectively, A is the effective guide wavelength, and h represents the free space wavelength multiplied by the ratio of the velocity of the electrons to the velocity of light.

4 To obtain proper phasing between adjacent gaps, it is then important that where n is an odd integer. In the preferred embodiment shown, it equals 1, corresponding to a separation of electron path and serpentine wave guide axis of Equation 2 provides a definite lower limit for the value of L1+Lz and indicates that the time of transit between successive gaps is greater than one-half period. This further implies that the transit time for each gap is a large fraction of a wavelength and the effect of field strength variations during transit must be accounted for. In particular, from these considerations, it can be shown that a maximum energy per gap occurs for a gap length g equal to .372)\ The distance between gaps should be as short as possible consistent with Equation 1 to increase the effective average accelerating field. This means that for maximum gain L1 and L2 should be as short as feasible.

In general principles of operation, this circuit resembles other wave circuits proposed for traveling wave tubes. The signal to be amplified is applied to the tube by way of the input 15 and progresses along the wave circuit in a serpentine path. At the regions of beam traversal, the interaction with the electron stream produces amplification of the stream.

By taking advantage of the symmetry of this circuit and providing two distinct electron streams, the beam current interacting with the wave can be increased in a manner that avoids the space charge defocusing which usually serves to restrict the density attainable in a single stream. Fig. 2 illustrates a microwave device similar to that shown in Fig. l modified by the addition of two more electron streams. Elements having a counterpart in Fig. l have been designated by the same reference characters. The second and third electron streams are projected along the electron paths 27 and 37 which are defined by the electron guns 21 and 31 and the collector electrodes 22 and 32, respectively. As, with the electron path 17, electron paths 27 and 37 extend parallel to the axis of the serpentive wave guide and are separated odd multiples of a quarter of a guide wavelength therefrom. In addition to providing greater gain, the additional streams can be utilized to provide a particular shaping of the gain vs. frequency characteristic of the tube. In particular by providing different accelerating beam voltages to provide different stream velocities, it is possible to staggertune the circuit to give a flatter band pass characteristic than is possible otherwise, compensating for the effect of the Waveguide velocity as a function of frequency on the gain'frequency characteristic of the amplifier. Alternatively the beam velocities may be staggered to obtain flatter gain vs. level characteristics. If one beam is adjusted to the velocity of maximum interaction, and hence highest gain, and the others at successively greater velocities, the first beam would be the main contributor to gain at low levels. However, at high levels where this beam would be slowed down to a less efficient velocity, since it is characteristic that, at high level operation, the velocity of the electron beam is slowed by interaction with the traveling wave, the other beams,'if properly adjusted, can be caused also to slow down in turn to the velocity of maximum interaction. Q

Similarly, additional distinct beams may "be provided either for achieving additional power or for realizing particular response characteristics. In this case, each of the additional beams is projected along a separate electron path, provision being made that each electron path is paralleltoand displaced an odd number of quarter wavelengths from the axis of the serpentine wave circuit.

asaotaac Fig. 3 shows an illustrative embodimentof atube-Which incorporates several features-of the present inventionfor mixing two separate input sign'alsfand "deriving; therefrom a desired modulation product-.. The wave circuit in this case includes three separate sectionsSl, 52-and 53, each of which is 'a wave' guide-folded back and forth on itself as-described above; the-three sections i being alignedwith one another in a manner to permit'their: being-electron coupledby a common electron stream Whiehlis projectedbetween the electron gun 41 andthecollector 42 along the'aelectron' path 43; In-the arrang'ementshown, the three wave circuit sections-are distinctand spaced apart byinsul'atingmaterial 8-1 to permit a diflerentbearn volt.-- age and hence electron st-ream'velocitythrough the separate sections, although arrangements. are po'ssiblein-which the electron velocity. should be-substantiallyv the same in the various sections, in which case; the-three-sections may bemade integral with one another and-maintainedat the same voltage. In the instant, case, thedifferent electron velocities are controlled by the voltage sourcesdl, 62 and 6'3. which provide the correspondingaccelerating potentials. The first signal to be mixed is applied'to the. wave circuit; section 51 by way of the -wave-guideinput 55- for travel therethrough. The parameters of sectionSl are adjusted to provide the desired interaction withthe electron flow in the manner described earlier'for'thetube of Fig. l. In particular, this section is positioned to have the electron path 43 parallel to and spaced one-quarter the guide wavelength of this first signal from the serpen-' tine Wave guide axis 91. As a result of the interaction, modulation of the stream results and there isgirnpressed on the stream the first input signal. To minimize undesirable reflection etfects -th e section 51 is terminated in an appropriate energyabsorbing element 71, and the coupling between sections is effected primarily by means of the electron stream, since the energy transfer byway of the electron beam apertures in the-guide walls is made small. The second input signal to. bemixed is applied to the second wave circuit section 52 by wayof the waveguide input 55. In this case; the parameters of section 52 are chosen to provide interaction between the electron stream and this secondinputwave. In particular, section 52 is positioned so that the electron path is displaced substantially one-quarter the wave-guide length of this second input Wave from the serpentine wave guide'axis 92 ofthis second section, Since the two inputsto be mixed presumably willhave different operating frequency bands, this will result in a displacement of axes 91 and 92 of the first two serpentine wave circuit sections. By way of'example, there is shown the case where the frequency of the second signal is less. Asa result of the interaction, the second signal will be impressed on the electron stream, which already has thereon the first signal, and cross. modulation results. As before, the. second section is terminated in anappropriate energy absorbing element'72 to minimize reflection efiects. The? modulated" electron stream continues along the electron path and the A.-C. convection current in the stream excites a traveling wave in section 53 which travels therethrough in the manner of an input signal. By proper adjustment of the parameters of this third section, a predetermined modulation product of the two input signals present in the electron stream can be chosen for selective amplification. In particular this third serpentine wave guide section should be constructed to have its axis 93 displaced substantially one-quarter'the guide wavelength of the desired modula tion product. In this way, when the other conditions defined by Equations 1 through 3 above are satisfied in the desired operating range, cumulative interaction will occur between the selected wave and the electron stream which will provide amplification to the wave. At the output 55 of this third section, there will be available the modulated wave for utilization.

It can be seen that the principles illustrated by the above arrangements may be utilized in various other em- 6 bodiments, In-particularalthough the first two-ar-range+ ments-have been described as'an amplifier, bysuitable modification a portion of the output wave energy'can' be returned'to the input for self-excitation and there is made available an oscillator. Additionally, it should be clear that the arrangement of Fig. 2 although described primarily as directed towards obtainingifiatter gain characteristi'cs, can be utilized to provide particular'selective characteristics,. both with respect to frequencyand also with respect to level, as for compressors or expanders. Similarly the principles exemplifiedybythe arrangement of Fig. 3 can be utilized in a variety of'ways.

What is claimed is: I a 1. In 'a microwave device which utilizes the interaction between an electron stream and a traveling wave to secure gain, a wave circuit comprising a wave guide folded back and forth on itself "a plurality of times forming a serpentine wave 'guide structure, and a plurality of electron sources,- each forming-and projecting an electron stream transverse to the folds of. the serpentine wave guide structure and; displaced an odd number of quarterwavelengths of the operating frequency-from the axis of the serpentine wave guide structure. I V

2. In an microwave'device which utilizes the interaction between a traveling wave and an electron stream to secure gain, a wave circuit comprising a wave guide folded back andfortli on itself a plurality of times forming a serpentine-like wave guide structure, a plurality of electron sources, eachforming and projecting an electron stream displaced an odd number of quarter wavelengths of the. operating fre uency from'the axis of the serpentine wave guide structure, and means including a voltage source connected between said wave guide and each source for providing substantially the same velocity to each of the: plurality of electron streams through saidfwave guide; 3.. In a'microwave device'which utilizes the interaction between a'travelingwaveand an electron stream to securegaima wave circuit comprising a wave guide folded back and'forth on itself forming. a serpentine-like wave guide structure, a plurality of-electron sources, each forming and projecting an electron stream through successive folds andalon'g an electrori path which is displaced an odd number "of quarter wavelengths of th e operating frequency onoppositesidesi of the axis of the serpentine wave guide structure, and beam accelerating means including a voltage source connected between each electron source and the wave guide'for providing a different velocity to each of the plurality of streams.

41 In a microwave device which utilizes the interaction between an electron stream and a traveling wave, a serpentine wave circuit for-propagating the: traveling wave, and a plurality of electron sources each for projectinga distinct electron stream, the various electron streams being spaced apart to traverse the wave circuit along separate electron paths spaced 'anoddnumoer of quarter wavelengths from the. axis of the serpentine wave circuit.

5. In a traveling wave device, an electron source for forming and projecting an electron stream along an electron path, a first wave circuit along said electron path supplied with a first signal of a first frequency for modulating the electron stream, a second wave circuit further along said path supplied with a second signal of a second frequency for providing a traveling wave therethrough for further modulating the electron stream, a third wave circuit along said electron path wherein the modulated stream excites a traveling wave whose frequency is a desired modulation product of said first and second frequencies, and means for deriving said last-mentioned traveling wave for utilization, each of the first, second, and third circuits comprising a wave guide folded back and forth on itself a number of times to form a serpentine wave guide structure which forms with said path of electron flow a plurality of successive interaction regions at the regions where the electron path traverses each fold of the wave guide, the axis of each of the serpentine wave guide structures being displaced from the path of electron flow an odd number of quarter wavelengths of the frequency of the wave propagating therealong defining short and long wave propagation paths between adjacent interaction regions, the length of each of said long paths in any one of said wave circuits being equal to the length of each of said short paths plus an integral number of wavelengths of the operating frequency in said one wave circuit.

6. In a microwave device which utilizes the interaction between an electron stream and a traveling wave to secure gain, a wave circuit consisting of one continuous wave propagation path folded back and forth upon itself a plurality of times forming a serpentinelike wave guide structure, an electron gun and a collector electrode positioned for defining an electron path transverse to the folds of the serpentine wave guide structure whereby a plurality of successive interaction regions are formed at the regions where the electron path traverses each fold of the wave guide, said electron path being parallel to and displaced an odd number of quarter wavelengths of the operating frequency from the axis of the serpentine wave guide structure defining short and long wave propagation paths between adjacent interaction regions, the length of each of said long paths being equal to the length of each of said short paths plus an integral number of Wavelengths of the operating frequency.

7. In a traveling wave device, an electron source for forming and projecting an electron stream along an electron path, a first wave circuit supplied with a first frequency signal transversely successively intersecting said electron stream at a group of first-signal electron-stream interaction points, a second wave circuit supplied with a second frequency signal transversely successively intersecting said electron stream at a group of second-signal electron-stream interaction points, a third wave circuit transversely successively intersecting said electron stream at a group of points where said electron stream induces in said third circuit a desired heterodyne modulation signal of the first and second signals, and means for connecting said heterodyne signal to an output, each of the first, second and third circuits comprising a wave guide folded back and forth on itself a number of times to form a serpentine Wave guide structure, the axis of each of the serpentine wave guide structures being displaced from the path of electron flow an odd number of quarter Wavelengths of the frequency of the wave propagating therealong defining short and long wave propagation paths between adjacent interaction points, the length of each of said long paths in any one of said wave circuits being equal to the length of each of said short paths plus an integral number of wavelengths of the operating frequency in said one wave circuit.

8. In a microwave device which utilizes the interaction between an electron stream and a traveling wave to amplify the wave, a wave circuit comprising a wave guide folded back and forth on itself a plurality of times forming a serpentine-like wave structure and an electron source and collector-electrode positioned for defining an electron path transversely through the plurality of folds of said serpentine structure whereby a plurality of successive interaction regions are formed at the regions where the electron path traverses each fold of the Wave guide, said electron path being parallel to and displaced an odd number of quarter wavelengths of the operating frequency from the axis of the serpentine structure defining short and long wave propagation paths between adjacent interaction regions the length of each of said long paths being equal to the length of each of said short paths plus an integral number of wavelengths of the operating frequencies.

9. In a traveling wave type device, first, second and third wave circuits, each comprising a wave guide folded back and forth on itself a plurality of times forming a serpentine-like wave guide structure, input means to each of said first and second wave circuits for applying for propagation therethrough waves of a first and second frequency, respectively, and an electron source for forming and projecting an electron stream in turn through the folds of said three Wave circuits for electron coupling thereof in the regions where the electron stream passes through the wave guide folds, the three wave circuits being positioned for providing an electron path through the serpentine wave guide structure of said first and second circuits parallel to and displaced from the axis of said serpentine structure approximately an odd number of quarter wavelengths of said first and second frequencies, respectively, and through the serpentine wave guide structure of said third circuit patrallel to and displaced from the axis of said serpentine structure an odd number of quarter wavelengths of a desired modulation product frequency of said first and second frequencies defining short and long wave propagation path between adjacent regions of traversal of the beam in each of said first, second, and third circuits, the length of each of said long paths in any one circuit being equal to the length of each of said short paths plus an integral number of wavelengths of the operating frequency in said circuit.

References Cited in the file of this patent UNITED STATES PATENTS 

